Glass substrates are widely used for displays of liquid crystal displays, organic EL displays, etc., substrates of hard disks, filters, sensors, etc., cover glasses of solid state image pickup devices such as CMOS, and the like. In particular, active matrix displays in which pixels are driven by active elements, typified by thin-film transistors (hereinafter referred to as TFT(s)) form the mainstream of liquid crystal displays and organic EL displays, and are widely used in the displays for displaying color images and movie images of liquid crystal televisions, notebook computers, LCD monitors, cellular phones, and digital cameras. In such active matrix displays described above, micron-order high definition electronic circuits such as TFT elements and signal lines are formed on the surface of a glass substrate using a thin film.
The glass substrates for use in the above applications require various properties described below (see Patent-Document 1):    (1) the glass substrates are required to have substantially no alkali metal oxide because when the glass has an alkali metal oxide, alkali ions are dispersed in a film-formed semiconductor material during a heat treatment, which leads to degradation of film properties;    (2) the glass substrates are required to have chemical resistance which is not deteriorated by chemicals such as various acids, and alkalis, used in a photoetching process;    (3) the glass substrates are required to have a high strain point in such a manner that the substrates are not thermally shrinkaged by heat treatment in processes of film formation, annealing, etc;    (4) the glass substrates are required to have a small density so as to achieve lightweight displays; and    (5) the glass substrates are required to be matched to the coefficient of thermal expansion of peripheral members.
In view of fusibility and formability, the following properties are also required in this kind of glass substrate:    (6) the glass substrates are required to have excellent fusibility so as to prevent fusion defects, which are undesirable for glass substrates, from occurring in the glass. In particular, there should be no bubble defects; and    (7) the glass substrates are required to have excellent devitrification resistance so that foreign substances generated during fusion and formation processes do not remain in the glass.[Patent-Document 1] Japanese Patent Application Laid-open No. 2000-302475